A harlequin fetus was born to unrelated parents with a negative family history for any form of ichthyosis. The child was grossly deformed, showing thick plaques on the cutaneous surface, and survived only 48 hours. Pathological studies of the skin showed marked thickening of the stratum corneum, but no other distinctive changes. X-ray diffraction analysis of the horny layer revealed the presence of a cross-β fibrous protein rather than the usual α-protein. The epithelial surface of other organs appeared normal. The thymus gland was involuted and showed changes in the corpuscles. Abnormalities were also observed in the conversion of cartilage to bone.